Aj. Conley et al., YOLK STEROIDS DECLINE DURING SEXUAL-DIFFERENTIATION IN THE ALLIGATOR, General and comparative endocrinology, 107(2), 1997, pp. 191-200
The leading explanation of temperature-dependent sex determination (TS
D) in reptiles postulates that (1) ovarian differentiation is directed
by estrogen and that (2) estrogen is synthesized in the developing go
nad following induction of aromatase expression. However, the source o
f steroid substrate for aromatization has not yet been identified. In
addition, sex ratios vary as a function of clutch, but such biases are
as yet unexplained. To address these issues, we measured estradiol, t
estosterone, and androstenedione in yolks of the American alligator (A
lligator mississippiensis) before, during, and after the period of gon
adal differentiation in this TSD species. Eggs were collected from a w
ild population in Louisianna and were incubated at male- and female-de
termining constant temperatures in the lab, as well as at intermediate
temperatures that produced both sexes. Steroids were assayed in yolk
extracts after celite column chromotography. All three steroids were f
ound to be in the range of nanograms/gram of yolk at stage 16. Androst
enedione was the predominant steroid, 2- to 3-fold higher in concentra
tion than estradiol and 15- to 20-fold higher than testosterone. The l
evels of these steroids declined (5- to 30-fold) between stages 16 and
25, most markedly between stages 21 and 23, regardless of incubation
temperature. The chronology of this sharp decline in steroid levels in
our study coincides with the timing of gonadal differentiation in thi
s species, between stages 21 to 23 based on previous reports. Estradio
l levels in yolks differed by 3-fold in some clutches relative to othe
rs, whereas, no clutch differences were apparent for either androstene
dione or testosterone. These data demonstrate that alligator yolk cont
ains high concentrations of two steroid substrates utilized for estrog
en synthesis, as well as significant quantities of estradiol itself. W
e hypothesize that estradiol levels in yolk provide a steroid backgrou
nd, variable among and within clutches, on which gonadal development i
s initiated and proceeds. As a consequence, we suggest that yolk provi
des an epigenetic maternal contribution that modulates the effect of i
ncubation temperature on hatchling sex. (C) 1997 Academic Press.